This invention relates to a driving circuit for driving a stepping motor, and more particularly to a stepping motor driving circuit which has a failsafe function for protecting the circuit in the event of an electrical fault in the circuit.
Conventionally, a stepping motor driving circuit is generally provided with a protective device which is operable to protect the circuit in the event of a short circuit between a circuit element and wiring or the like, by preventing the stepping motor and/or circuit elements from being damaged or broken by excessive current (overcurrent) flowing in the motor and/or the circuit elements.
The conventional protective device includes the following types:
i) a type employing a fuse which is cut by overcurrent;
ii) a type employing a resistance having a low resistance value and arranged in the driving circuit at a suitable place for detecting overcurrent such that a relay or like means is actuated upon detection of overcurrent by the resistance to cut off the supply of driving current to the stepping motor; and
iii) a type having means for applying driving pulses to the stepping motor and detecting the rotational angle of the motor driven by the pulses, means for detecting a fault in the driving circuit based upon the detected rotational angle and the number of driving pulses applied to the motor; and means for cutting off the supply of driving current to the motor upon detection of a fault.
The above type i) suffers from low responsiveness, and the type ii) has a drawback of large heat loss due to the use of a resistance. On the other hand, the type iii) is not capable of selectively detecting only electrical faults, since it can erroneously judge that a fault has occurred, even when the means for detecting the rotational angle becomes faulty, when the motor skips, or when there takes place a mechanical fault.